Infantry under 50% of army? Never! (2,4)
I believe the answer is:
my foot
'never' is the definition.
(both can mean express disbelief)
'infantry under 50 of army?' is the wordplay.
'infantry' becomes 'foot' (foot soldiers).
'under' means one lot of letters go next to another.
'50 of army?' becomes 'my' (I can't explain this - if you can you should believe this answer much more).
'foot' put after 'my' is 'MY FOOT!'.
(Other definitions for my foot that I've seen before include "bunk!" , "I've some nails here?" , "I stand on this" , "Definitely not" , "Nonsense!" .)